A wide array of intrusion detectors are known in the art. Some of these detect the presence of an intruder in a particular area and others detect intrusions into the area, or attempts to break into the area. One type of intrusion detector for determining break-in is a glass breakage detector.
One type of glass breakage detector analyzes sounds picked up by a microphone to determine if they are produced by breaking glass. A foolproof determination of glass breakage by acoustic means is extremely complicated since many factors must be taken into account in order to avoid both false alarms, when there is no break-in, and undetected events of true glass breakage.
U.S. Pat. No. 3,863,250 to McClusky, Jr. describes a glass breakage detector which is directly mounted on a sheet of glass whose breakage is to be detected. The detector comprises a sensor mounted on layers of material which attenuate acoustic frequencies which are not characteristic of the shock of breaking glass.
U.S. Pat. No. 4,134,109 to McCormick et al. comprises a signal analysis circuit which utilizes a sound having an intensity above a given threshold level to start the detection process. The system waits a predetermined interval and then determines if the integrated signal at a majority of a plurality of frequencies characteristic of falling glass is above a threshold during a pre-set time window starting after the interval. If the threshold condition is met and the sound at these frequencies ceases by a pre-set time, an alarm is sounded.
U.S. Pat. No. 4,668,941 to Davenport et al. describes a glass breakage detection system that utilizes the frequency components of the thump of glass breakage at about 350 Hz and the tinkle of breaking glass caused by collision of glass fragments at about 6.5 kHz. A very low frequency signal triggers a time delay of about 200 milliseconds and establishes a time window which closes at 800 milliseconds or one second. An alarm is sounded if there is a high frequency signal greater than a threshold value during the time window. In order to avoid false alarms such as may be caused by tapping on the window, a particular frequency to voltage convertor is used to exclude all frequencies below 4.5 kHz.
U.S. Pat. No. 4,837,558 to Abel et al. describes a tuned unidirectional glass breakage detector responsive to sounds in the 4 to 8 kHz range.
U.S. Pat. No. 4,853,677 to Yarbrough et al. describes a glass breakage detector which detects sounds at 3 kHz to 4 kHz to determine if there has been glass breakage. The detector also includes a door or window opening detector which detects pressure changes at 1-2 Hz. The sensitivity of the glass breakage detector is increased in the presence of low frequency signals since the combination is said by the patent to indicate a break-in wherein steps have been taken to minimize breaking glass sounds.
None of the above prior art devices is sufficiently effective in determining glass breakage for certain types of glass such as safety or laminated glass. Furthermore, the analysis of sounds provided by these devices is not capable of determining glass breakage for a variety of glass types while also having a low false alarm rate.
There are also known methods of supervising the operation of an audio intrusion detection system. For example, there are methods for detecting attempts to inconspicuously tamper with the audio detection system. U.S. Pat. No. 5,164,703 describes a supervisory circuit which periodically generates a test sound into the space monitored by the audio detection system. The detection system detects reflections of the test sound in the monitored space and generates a corresponding test signal. Using a comparator, the test signal is compared with a predetermined threshold and the operability of the detection system is determined based on the comparison results. The intrusion detection mode of the system is inoperative during the supervisory time periods. It is noted that the threshold used by the comparator of the supervisory circuit is constant.